Many electron spin resonance (ESR) studies of irradiated organic crystals have shown that the environment of the damage center dominates chemical reactions involving the damage molecule: for example, these reactions are stereospecific. Thus detailed knowledge of the structure and the unpaired electron distribution in the environment will be important for understanding radiation damage in irradiated organic compounds and biological materials. For this reason, we intend to study the environments of damage centers in x-ray irradiated organic compounds. The primary experimental tool to be employed is "negative ENDOR," that is, a new technique of electron-nuclear-double resonance (ENDOR) which our group has developed recently. It has been shown that negative ENDOR allows one to obtain a significant knowledge of the environment of a damage center. The present project is intended to study the case of L-alanine because of the hydrogen exchange reaction which is apparently similar to that found in irradiated biological materials. Using many deuterated crystals of L-alanine, proton-deuteron exchange at the radical environment will be observed as a function of the temperature and period of heat treatment of irradiated crystals. Then, the detailed mechanism of the hydrogen exchange reaction will be clarified.